1. Field of the Invention
This invention relates to a special method for the turning of workpieces and preferred applications of the method. The invention also concerns a screw-in type artificial hip joint socket designed for cement-less implantation in the human medical domain.
2. Description of the Related Art
The principle of conventional lathing is a method which has been known of for many years and is used for the cutting manufacture of workpieces, e.g. of wood, metal or plastic. In recent years, lathing technology has undergone rapid advance due to the introduction and continuous development of numerical controls. Thus, today it is absolutely no longer any problem to, for example, maintain a constant cutting rate along a surface contour. With a suitable program it is now relatively simple to produce even the most complex rotationally-symmetric geometries in very short machining times. Furthermore, machines of this type can be further upgraded by equipping them with tool drives because this allows even complex workpieces to be lathed and milled to form a finished product with a single clamping. Despite this, there are certain limitations in connection with certain geometrical shapes or because of the time required. It is for example a fact that lathing in general has considerably shorter machining times than does milling. In addition, turning yields better surface qualities. If as a result of the geometry of a workpiece it is only possible to employ milling techniques, it is unavoidable that either a considerably longer machining time is involved or that a less uniform surface has to be accepted. However, this notwithstanding, even milling techniques are subject to certain limitations as far as the geometry is concerned. Thus, for example any corner of a milled contour in the radial plane of the milling axis can never have a corner which is sharper than the radius of the milling tool used. And while it may be possible to produce sharper contours using techniques such as broaching, percussion and erosion, it is necessary to transfer the workpiece to a different machine for this end. In the case of erosion the time requirement is also extremely long. While it is also true that the cutting of non-circular contours has been possible for a number of years now using profiling turning devices available commercially, these devices are expensive and therefore require a corresponding scale of capital investment. Furthermore such machines can only be connected to the initially intended interface and are limited to the specified contour with two-dimensional non-circular geometry.
In the past there have been attempts to enable lathes to machine non-circular workpieces by fitting special mechanical modules. One machine of this type is proposed in the German publication DE 25 15 106. In addition to the very complex and very sensitive mechanical configuration, this machine has extremely limited possibilities which in turn are themselves limited to the generation of two-dimensional non-circular geometries.
The geometrical possibilities for non-circular machining can be expanded with respect to a tool which can be fitted to the lathe if for example the cutting drive can be controlled in a freely programmable fashion. A tool of this type is for example described in the German publication DE 35 09 240 A1. In this case piezoelectric or magnetostrictive actuators are used in order to achieve a dynamic shift of cutting relative to the workpiece using appropriate electronic controls. However, this technique only allows extremely small adjustments to be achieved. While it would be technically possible, for example, to use a magneto-dynamic system to achieve considerably larger control movements, these would as previously be limited to a single movement axis. In order to achieve specific three-dimensional discontinuous machining it would be necessary to add a second or possibly even a third orthogonally arranged movement unit to create a tool with complex directions of movement, whereby this would be of extremely complex design and demand highly sophisticated control electronics. To date a tool of this design is not yet available.
There are known other special turning lathes which have been developed for non-circular machining, for example, of pistons for internal combustion engines. Modern pistons have in fact a very slight oval cross section, generally elliptical, in order to compensate for anisotropic expansion during heating. Having said this, there is however only a very slight deviation from the circular shape, whereby the contour also has a very flowing shape. There are no jumps or extreme discontinuities present. This being the case, the constructional design of a machine with this capability does not represent any major difficulty. In principle it is sufficient to allow the tool to oscillate with a slight amplitude on the X-axis of the diameter whilst the carriage traverses the workpiece in the Z-axis. In so doing the path of the tip of the tool will follow a more or less sinusoidal curve such that extreme acceleration is not necessary. This latter would be very difficult to achieve despite the reduced mass of the system. It is pointed out that such machines require a coupling of the workpiece rotation to the movement along the x-axis whereas the advance in the Z-axis can be freely chosen. In fact the generation of the non-circular contour is restricted to the two-dimensional diameter plane and is only extended in a third dimension by way of the Z-axis. In reality the Z-axis is not actually involved in the generation of the non-circular contour. There is no technique for moving the carriage along the Z-axis in jumps or with superimposed oscillation, for example.
A special machine of the type described above is described in the German publication DE 40 31 079 A1, for example. In this case it is proposed to control the drive required for the oscillating movement of the tool (for example an electric linear motor or a hydraulic system) by means of an extra computer control in addition to the existing mechanical control, whereby this could be a personal computer, for example. However, a machine of this description would be limited in its possibilities to the intended and similar applications unless its basic kinematic method is modified. Furthermore, a special machine of this description would be relatively expensive to acquire.